The present invention relates to an automatic tilt angle compensator, which is used in survey instruments, measuring instruments, etc. to measure changes in the amount of tilting, to maintain an optical axis of an equipment in the vertical direction or in the horizontal direction or to form a light beam irradiation plane in the horizontal or vertical directions.
When there is provided an optical member, in which transparent liquid is sealed in a sealed container and two layers of gas and liquid are formed in the sealed container and in case the entire system including the optical member is tilted, the liquid is turned to wedge-like form. The wedge-like form is deformed at a constant rate with respect to the tilting of the system. If a light beam is passed through the liquid in wedge-like form, the liquid with its form changing at a constant rate with respect to the tilting of the system plays a role of a prism. As a result, an optical axis of the light beam after passing through it forms a deflection angle at a constant rate with respect to the tilting of the system. An optical axis compensator based on this principle is disclosed in each of JP-A-8-75466 and JP-A-7-19871.
The sealed container as described above contains a transparent member to allow the light beam to pass. In the automatic tilt angle compensator as described above, the transparent member of the optical member is immersed in or emerged from the sealed liquid depending upon the condition of the automatic tilt angle compensator. For this reason, even in case the transparent member is emerged from the liquid, the liquid remains and is attached on the transparent member in the state of liquid drops or liquid film. This leads to a problem that the light beam passing through the transparent member diffuses.
An optical axis passed through the liquid in wedge-like form is changed with respect to not only the change of tilting the system but also the change of liquid temperature according to environmental temperature. Because the change of liquid temperature causes the change of a refractive index of the liquid. Therefore, the change of optical axis with respect to the tilting of the system differs according to temperature condition.
In the conventional type automatic tilt angle compensator, the use of the compensator is limited to the case where the entire system is installed in almost horizontal or in almost vertical position and it is difficult to use the compensator in both conditions at the same time, i.e. in almost horizontal and in almost vertical conditions. To use the compensator in both conditions, it is necessary to use it by attaching an optical system separately prepared or the sealed container must be made movable in advance or must be assembled to suit the use at almost horizontal position or at almost vertical position. In these cases, reproducibility at the removal or assembling of the optical system is low, and accuracy is not satisfactory. Also, the cost of the parts, these parts demand high accuracy, increases.